Method of operating an iron blast furnace



Dec. 31, 1957 c. E. AGNEW METHOD OF OPERATING AN TRON BLAST FURNAOE Filed June l2, 1952 5 Sheets-Sheet 1 luwabtou bien INVENTOR.

CHAR/.Es E. AGNEW' MJL/v v ATTORNEYS $.55 .2.6 W tra BY ,W

@l -mmm INOZ NQLLVUYdJUd muy United States Patent O METHOD F UPERATING AN IRON BLAST FURNACE Charles E. Agnew, Shaker Heights, Ohio Application .lune 12, 1952, Serial No. 293,141 9 Claims. (Cl. 'i5- 41) This invention relates to anl improved method of operating an iron blast furnace so that oxygen enrichment of the air blast. may be continuously used advantageously.

An object of the present invention is to provide a method of operating a blast furnace producing metallic ir'on chiefly from iron ores and wherein the furnace is continuously burdened with iron bearing materials, uXing materials and carbonaceous solids so that the natural temperature head developed in the lower boshy section of the furnace, in relation to the tapping slag temperature, is insuicient for continuous furnace operation using natu-ral. air blast, and then develop the necessary head by increasing heat generation. from carbon combustion. per unit of air without increasing the Volume of the blast entering the tuyeres or substantially increasing gas volume leaving the coke combustion zone.

Another object of the presentinvention is to provide a method of operating an iron blast furnace, continuously burdened as defined above, and wherein the burden isi so selected with regard to chemical composition that the' slag forming constituents provide, approximately' at the section of the furnace just above the fuyere level, a slag. having a melting temperature not naturally hot enough to assimilate the coke ash released by coke carbon conbustion at the tuyeres and still be free-running'k at tapping, and then enriching the' air blast with oxygen' to generate the heat' volume necessaryv for coke ash assimilation While retaining, the free-running property of the slag.

Other objects and' advantages of the present' invention willv be apparentI from the' accompanyingv drawings and description and the essential features thereof: will be" set' forth in; the appended claims'.

In the drawings,-

Fig. 1-l is a diagrammatic view of a'` blastl furnace indicating' the operations which` occurvr at' variousv zones;

Fig. 2 isf a; graph; of slagtemperatures in' a blast furnace under certain conditions;

Fig; 3y is a diagrammatioview ofablast furnace-incentral sectional view showing, in-` broken lines two temperature zones under diierentconditions; while Fig. 4 is a graph ofslagteinperaturesina blast furnaceunder conditions differingfrom those of Fig. 2.

An iron blast furnace operation has two divisions of work, as illustrated in Fig. 1. Those sliilled in the" ar'tv understand that the furnace is continuously burdened by the* introduction' of suitable amounts' of ironv hearing' rnaf ice tenais, uxing stone, usually lime 'stone or dolomite, and carbonaceous solids, usually coke,` into the top ofthe furnace. The upper portion df the furnace above the line A in Fig. 1 isz usually known as the shaft where these raw materialsr are preparedfor smelting, which work consists of eliminating all of the volatile matter in the raw materials, heating the non-volatile material to fusion temperature, and indirect reduction of iron oxidel by means of carbon monoxide in gases moving' upwardly. The portion of the furnace below vthe line A comprises mainly the bosh and hearth and the work performed` there consists in smelt-ing; the already prepared materials into iron and slag, which work consists of separating slag' forming constituents froml iron forming constituents;` and alloying desired amounts of other metallic elements with the metallic iron to produce a pig iron product having a specied chemical composition. The slag is periodically tapped through the cinder notch 10 and the iron is peri@ odically tapped through the tapping hole- 11. A blast of air u-nderpressure is continuously introduced at the tuyeres 12 of which there are' a number around the circumference of the furnace. This air is preheated in stovesnot shown. An intense combustiony zone indicated roughly at 13 occurs near eachA ItuyereAWhere,l through the exothermic reaction C +O2=CO2, the coke carbon is burned by means of the air oxygen to carbon dioxide (CO2) creating14,580 B. tru. per pound of carbon. Alirnost immediately, throughv the endothermic reaction- CQ'2-,l-C=2CO`, the CO2 gas reacts with incandescent coke adjacent to the combustion zone to form carbonmonoxide (VCO) so that the net result of these thermal reactions is' the production of 4370 B'. t.l u'.A per pound of.v carbon consumed. l l l y The workl of both divisions of the blast furnace is effected through generation of heat and the application of it to the raw materials, thesessential difference betweenv the' two" being that the shaft or preparation work is; or can be, effected at relatively low temperatures- (below fusion), and `the bosh and hearth work of smeltin'g is; and must be", effected at high temperatures (above fusion). Obviously', there must! be equitable division of heat be` tween th`e two divisions of work with ideal proportional division' being determined by the needfor hea-tvolume in the' respective divisions of worlc. Obviously, material in the shaft of the furnace cannot be lprepared faster than it can be smelte'd in the bosh and hearth. Conversely, the bosh' ,andl lea'r'tli` cannotv smelt materials faster than they can' be' prepared in the shaft. In short,f equilibrium between the two divisions of worli must be maintainedV inorder to' operate the furnace. Since thermal requirements ofthe respective' divisions of; work vary with the charac-V ter of raw materials used, i'tis' inevitable that usual-'ly there will beI reserve capacity in one of the divisions of Work which cannot be' utilizedy undernatural operati-ng conditions. To utilize this reserveV capacity,` unnatural operating conditions must be created which are favorable.l to its use. v My invention teache's'hou'r` this may be: done.

The" cubical dimensions of the shaft,4 and of the. bosh and"hearth',I of al blastf'ffnac'e are' coni'r'rio'nly considered to be fue Semana of' furnee abuveaud'eiow a non.

zontal plane located 'at the mantle. This is the location of the structural steel ring at the upper limits of the bosh and at the greatest diameter of the furnace where the weight of the shaft is supported. However, it is reasonable to say that the true dimensions of these respective divisions of the furnace are not determined by fixed construction lines, or by a given horizontal plane, but by variable lines and planes determined by the thermal conditions created within the furnace by the character of the raw materials charged and the practice methods used. lt is herein assumed that a furnace is designed so that the shaft preparation work occurs above approximately the line A of Fig. 1 a little above the upper limits of the bosh and that section of the furnace below that line is utilized for smelting. Specifically, since the smelting work begins with fusion ofthe raw material solids, the true top plane of the working bosh is always where th temperature is high enough to cause fusion.

The gas generated in the coke combustion zone 13 is the agent for the mechanical distribution of heat through` out the stock column above it. Heat is transmitted from the gas to the stock of raw materials through gas-solid contact while the gas passes through the interstic'es of the stock column. For any given weight of gas, its vol urne and velocity through the furnace will vary with its temperature and density. It should be quite obvious that the hotter this gas becomes, the greater will be its velocity through the shaft, thus cutting down the time it is in contact with the solid material to effect preparation work, and also for carrying greater volumes of loose material out the top of the stack. Obviously also, high temperatures may be carried so far up into the shaft that they have a bad effect on the top structure of the furnace. This occurs quite commonly when furnaces are being pushed for high production and excess water must be charged with stock to protect the top structure against these high temperatures under such conditions.

I have mentioned above the necessity of maintaining a balanced condition between the capacity of the shaft for preparing material for smelting and the capacity of the bosh and hearth for smelting such material. Obviously, the productive capacity of the entire furnace must inevitably be governed by the division of the furnace having the lesser capacity. For any given weight of raw materials charged into the furnace, the ratio of thermal requirements for shaft low temperature work and for bosh and hearth high temperature work does not vary with the rate of stock travel through the furnace, but the thermal requirements for the respective divisions of the work per unit of time increases in direct proportion to the increase 1n stock travel rate through the furnace because this reduces the time available for effecting the necessary thermal-chemical reactions.

I am aware that it has been attempted to introduce oxygen with the air blast at the tuyeres in an iron blast furnace in an attempt to increase the production of the furnace. To the best of my knowledge, this has never been done successfully continuously in an iron furnace. I believe these failures have been due to the generation of greater heat Volume in the bosh and hearth of the furnace by the use of oxygen without providing means for equitable division of the increased heat volume generated so that the greater heat volume provided in the hearth and bosh could be advantageously used. It is the object of my invention to show how this may be accomplished.

A common condition met with in American blast furnace operation which decreases the preparation capacity of the shaft is the practice of continuously burdening the furnace with materials which result in the formation of slags of higher melting temperatures than are necessary for ideal furnace operation. It is common operating knowledge that the temperatures in zones of a furnace where slag formation occurs must be maintained Sulliclently high to permit the free-running of the slag formed downwardly through the furnace andA to permit Vthe tapping of the slag from the furnace in a free-running condition. Hereinafter in the specification and claims I have dened the temperature head as being the difference between lower bosh temperature and initial temperature required for slag formation on the one hand, and the difference between bosh temperature and tapping temperature of the slag on the other hand. Actual operation indicates a ratio of 1.10 or 1.15 to 1.00 between bosh temperature of the slag and initial fusion or tapping temperature of the slag as an ideal because it provides a safe margin of protection for slight variations in bosh and hearth thermal requirements caused by slight variations in raw material properties without losing the critical hearth temperature which is necessary to maintain slag in a free-running condition. Also, it assures minimum velocity of gas volume leaving the bosh and passing up through the shaft, consistent with the maintenance of temperature required for the completion of bosh and hearth thermal-chemical reaction such as the reduction of silicon, manganese and phosphorous. Gas rising from the bosh to the shaft is the agent which delivers heat from the bosh to the materials in the shaft. The temperature of the gas leaving the bosh must be the same as the temperature of the solids and semi-solids material contained in the bosh. There fore, the volume of heat drained from the bosh and hearth operation by this leaving gas will vary with the weight and volume of the gas and its temperature. Formation of slag in the bosh which requires high temperature for formation will therefore result in gas of high temperature passing upwardly in the shaft with resultant increase in pressure and velocity with 'a bad effect on the low temperature preparation capacity of the shaft as was previously pointed out. Also, if the tapping slag is hotter than necessary for furnace operation, more heat is drained from the furnace in the tapping slag than is necessary, thus increasing the thermal consumption of the furnace for a given production of iron. l

U. S. Bureau of Mines research indicates (Technical Papers 391 and 397) that slag is formed in three stages as follows. First, initial formation stage which comprises the initial segregation of the slag forming constituents contained in the ore and uXing stone, before the reduction of any silica (SiO2) to silicon (Si) and before assimilation of any coke ash. Second, the basicity stage which is the initial slag minus part of the silica reduced to silicon and plus a minor part of the coke ash. Third, the tapping stage which includes the slag forming constituents from the ore and stone after the reduction of the required amount of silica to meet the silicon specification for the iron plus all of the slag forming constituents from the coke ash.

It has been assumed herein that the composition of the slag in its basicity stage will vary from a higher basicity where none of the coke ash has been assimilated to a lower basicity where approximately 24% of the colte ash has been assimilated. It is well known that these variations do occur in the bosh of the furnace, due to variation in silicon content specified for the iron product and to the amount of coke carbon consumed above the tuyeres.

The composition of blast furnace slag, as determined in the chemical laboratory, is determined and reported as percentages of oxides, the four principal ones being silica (SiOg), alumina (A1203), lime (CaO), and magnesia (MgO). Those skilled in this art will understand that the silica and alumina come substantially from the gangue of the ore and the ash of the coke while the lime and magnesia come substantially from the fluxing stone, the commonest -of which is limestone or calcite having about 45 to 50 percent CaO and 0.5 to 5.0 percent MgO; or dolomite commonly having lapproximately 30% CaO and 21% MgO. Actually, the `slag consists of minerals compounded from various percentages of the four principal oxides listed above.

Nine minerals commonly encountered in coke tired blast furnace slag are listed: Forsterite (magnesium orthosilif 5 cate) will be encountered where slag contains low percentages of alumina and high percentages of magnesia;

Obviously, slags containing large percentages of calcium orthosilicate, magnesium orthosilicate and/or free magnesia, will have major influence on slag temperature, consequently, their existence in slag can be favorable or unfavorable to furnace operating economy depending upon the stage where they exist. Specific illustrations may be given.

Table No. l shows ideal tapping slag chemical composition. When calculated back through its stages of formation the respective mineral compositions indicate no calcium orthosilicate in the initial or tapping stages but an average of 27.27 percent in the basicity stage. With indicated mineral composition of the three stages there is low initial, higher basicity, and low tapping temperatures-conditions most favorable to furnace operating economy. Fig. 2 illustrates these conditions graphically.

Table No. 1

Slag No. Slag No. Slag No. Slag No. 1, Initial 2, Maxi- 3, Mini- 4, Tapfnrmnfinn mum bamum baping stage sicity slcity stage stage stage Slag chemical composition: Percent Percent Percent Percent SiO; 37. 22 32. 86 33. 59 35. 00 10. 26 11.00 12.05 15. 00v

Above constituents calculatedto 100% compositlon:

SiO 38. 37 33.88 34. 63 36. 0S A120 10. 58 11. 34 12. 42 15. 46 CaO 46. 05 49. 41 47. 64 43. 30 5. 00 5. 37 5.31 5. 15

Viscosity: number of paises-. 3 -2 -2 4 Tetrahedron (McCaffery) 6 7 8 6 Slag mineral composition: Percent Percent Percent Percent Akermanite 33. 81 31. 35. 90 8 15. 0B 15. 94 46 30. 50 33. 40 41. 58 Tricalcium disilicate. 22. 63 11. 60 5. 63 Calcium orthosilicate. 35. 45 19. 10 Monticellite 3. 02

Calculated average melting temperature:

Centigrade, degrees 1, 511 1, 737 1, 632 1,528 Fahrenheit, degrees 2, 752 3, 159 2, 950 2, 782

Table No. l assumes that the furnace was burdened on the basis of 30,000 pounds of ore to 6,400 pounds of limestone to 15,300 pounds of coke. The ore is assumed to contain 8% SiO2, 2.20% A1203, .10% CaO and .05%

MgO, and the balance iron, manganese, and phosphorous oxide. The stone is presumed to--analyze .84% 810 .24% A1203, 45.5% CaO, 4.75% MgO and the balance CO2. The coke is presumed to contain 3.71% SiOz, 2.64% A1203, 0.52% CaO, and 0.26% MgO. The slag chemical composition listed at the top in Table No. 1 adds up to 97% of the slag weight, 3% being allowed for MnO, FeO, CaS and alkalies. In the second list of the four constituents in Table No. 1 the constituents first listed have been recalculated on the basis of 100%. This 100% composition has then been referred to the McCaffery diagrams illustrated and described in Technical Paper No. 19, 1927Y Transactions of the American Institute of Mining and Metallurgical Engineers under the title Composition of Iron Blast Furnace Slags," in the 1931 Year Book of The American Institute of Mining and Metallurgical Engineers, and other A. 1. M. E. TechnicalPapers by McCaffery. For instance, in slag No. 1, taking the percentage of the four constituents listed as totaling 100%, and following the McCalery diagram, one nds that the mineral is in tetrahedron 6 which gives a slag mineral composition of the four constituents listed below the numeral 6 in slag No. 1, namely, akermanite 33.81%, calcium bisilicate 15.08%, gehlenite 28.46% and tricalcium disilicate 22.63%. Calculating the average temperature from these constituents with their respective melting points and prorating each percent according to its melting temperature one arrives at the calculated aver age temperature of 1511 C. or 2752" F. The other slags Nos. 2, 3 and 4 of Table No. 1 are calculated in the same manner to obtain their average melting temperature.

Slag No. 2 in Table No. l is assumed to be slag No. 1 with 87% of the 517 pounds of silica reduced to silicon to enter the iron and with the pounds of A1203, CaO and MgO otherwise the same as in slag No. 1. Slag No. 3 of Table No. l is assumed to have the silica the same asin slag No. 2 plus 24% of the silica in the coke ash. Likewise the other three oxide constituents of slag No. 3 are each assumed to have increased by 24% of the same oxide in the coke ash. Slag No. 4 of Table No. 1 is assumed to be slag No. 1 minus the amount of silica reduced to silicon to enter theiron and plus 100% of the coke ash slag constituents.

The calculated average melting temperature of the tapping slag (No. 4 slag), listed in Table No. 1, checks quite closely with the melting temperature of actual slags of the composition listed as measured in actual furnace operation.

If a furnace is burdened as taught in Table No. l, with a normal volume of blast air, the furnace can have a fusion zone not substantially above the dotted line of Fig. 3. However, it is now general practice to overblow such a furnace in order to increase production and this brings about the condition shown in the dot-dash line of Fig. 3 where the high temperature of the fusion zone is carried well up into the shaft of the furnace. This provides a very hot cone, sometimes as high as 1700" C., (U. S. Bureau of Mines Technical Paper 442) which takes up as much as 15% of the raw material low temperature preparation zone of the shaft. Utilizing my teaching, one may increase the low temperature preparation capacity of the furnace shaft by decreasing blast volume and adding oxygen to enrich the lessened natural air blast volume and still keep the fusion zone in approximately the dotted line position of Fig. 3, without sacricing stock travel rate through the furnace or productive capacity of the furnace. 'This is due to the fact that the heat volume generated per unit weight of air used, increased 4.34% for each 1% substitution of oxygen for nitrogen, but the gas Weight produced with the substitution increases only .0064%. Thus, one can greatly increase the heat volume generated for smelting work in the bosh without increasing the weight of heated gases carried up into theT shaft to any appreciable degree.

r Table 2A Low alumina Lake ore operation Slag No. 1, Slag No. 2, Slag No. 3, Slag No. 4, 5

Initial Maximum Minimum Tapping formation basicity basicity stage stage stage stage h 1 i. Sligoiclzemma compos Percent Percent Percent Percent 35. 77 33. 78 34. 74 37. 26 3. 94 4. 07 48 9. 67 47. 27 48. 80 46. 90 4l. 45 l0. 53 l0. 87 10. 41 9. 15

10o. oo 100.00 10o. o0 100.00 20 Viscosity: number of poises 2 2 2 2 Tetrahedron (McCaf H fery) 14 l Sla mineral com ositigon: p Percent Percent Percent Percent Alrermanitc 24. 2 17. 40 G3. 42 Calcium bisi1icate Anorthite Gehlenite Calcium ortho i icate 36. 91 47. 33 36.07 30 Tricaloium disilicato l. 03 28. 00 40. 82 3l. 42 Magnesia .63

Calculatted aveage melting empera ure: r Centigrada, degrees. 1, 732 1, 815 ll, 13o 1, 49a Fahrenheit,degrees 3, 150 3, 290 .5, 151 2 723 Table 2B 100% Dclomito Stone Low alumina Lake ore operation Slag No. 1, Slag No. 2, Slag7 No 2, Slag N o. 4,

Initial Maximum Minimum Tapping formation basicity basioity stage stage stage stage S1 chemical com osiiiim: p Percent Percent Percent Percent SiO: 35. 33. 80 34. 37. AlgOa- 3.94 4.07 5. 4S 9. el CaO- 36. 38. 00 57 32. 45 MgO 20.90 21.59 .0. 67 18.15 50 Above constituents calculated to comi n: possi.. 30. 7l 34. 68 35. 65 38. 20

4. 05 4. is s. s2 e. 92 55 37. 81 38. 99 37.52 33. 27 21.43 22.15 21.21 18. 01

Viscosity: number of l poises 1 1 60 Tetr'ahedron (McCafery) 14 14 5 ine al com osi- Shtinilzl r p Percent Percent Percent Percen Akermanite 4. 56 1. 91 Calcium bisilicatc Anorthite Magnesia Calculatedv average melting temperature:

Centigrade, degrees.

Fahrenheit, degrees. A

Tables Nos. 2A and 2B provide a comparison of slag mineral composition and thermal effects therefrom for' a furnace operation using relatively low alumina content Lake ores and, (2A) partial dolomite stone flux, and (2B) total dolomite stone uX. Fig. 4 illustrates the conditions graphically. The calculations of Tables Nos. 2A and 2B are carried out as described in connection with Table No. l.

As indicated, with partial dolomite ux, a high percentage of calcium orthosilicate is formed in the initial stage of slag formation, and with it high initial slag tempcrature, unfavorable to gas disposal from bosh to shaft. The basicity stage has a high temperature and the tapping stage has a low temperature but these normally desirable conditions have little value because of the excessively high initial stage temperature and the adverse effect from it on shaft raw material vpreparation capacity. Thermal conditions within the shaft are indicated by the high formation temperature of slag No. l in Table No. 2A, using 75% calcite and 25 dolomite. This high temperature requirement for the initial stage of slag formation would inevitably aggravate and accelerate the carry up of high temperature into the shaft as in thc dot-dash line of Fig. 3. The true cubical dimensions of the-Working shaft, and its capacity for performing low temperature raw material preparation Work, is reduced and the capacity of bosh and hearth operating `for performing high temperature smelting work is increased. This indicated change in ratio between shaft and bosh-and-hearth cubical dimensions from the commonly accepted dimensions above and below the mantle line A of Fig. l, show how a lessening of the preparation capacity of the shaft governs productive capacity of the combined divisions of the furnace.

With 100% dolomite stone (Table No. 2B), the percentage of calcium orthosilicate in slag of initial forma` tion is greatly reduced compared to the 75 calcite, 25% dolomite stone, and with it there is low initial slag formation tcmpcrature-favorable to gas disposal from bosh to shaft. Basicity temperature is reduced 9.92%, which although not desirable is less undesirable than the excessively high initial temperature with 75% calcite, 25% dolomite stone. Tapping slag temperature is reasonably low and therefore favorable to conservation of heat in the furnace.

Since theoretical calculation of thermal conditions within the furnace, presented herewith, checks actual measurements of those conditions with reasonable accuracy, like conditions for any furnace operation, and/or burden materials, can be predicted with reasonable accuracy. With reasonable prediction assured, unfavorable thermal effects Within the furnace can be anticipated and avoided by burdening the furnace with slag-forming constituents in proper ratio to each other to create preferred efects within the furnace and thus provide means for exercising a substantial degree of control over ratio of stock preparation and smclting capacities for any furnace operation.

With Table No. 2B, furnace operation using 100% dolomite stone, the plane where fusion would begin would be located approximately along the dotted line of Fig. 3, and with it material preparation capacity of the shaft would be increased approximately 15.00% compared to the operation using 75% calcite, 25% dolomite stone, thus increasing the capacity of the shaft to prepare stock relative to the capacity of the bosh 4and hearth to smelt stock.

Using average basicity temperatures of the two different sets of slags of Tables Nos. 2A and 2B, as indicators of heat volume concentrated in the lower bosh (slag weights being equal), the 100% dolomite slag would hold 9.92% less heat than the 75% calcite, 25% dolomite slag. Since the work to be performed inthe lower bosh (assimilation of coke ash) would remain the same with either slag', the reduced volume of heat in the lower bosh would have to be restored.

Since there is an indicated increase of 15.00% in low temperature material preparation capacity, and 9.92% reduction in smelting capacity, increased weight of burden could be prepared provided enough hearth heat could be supplied to smelt it. From the premise that successful use of oxygen-enriched air for iron blast furnace operation is contingent upon bosh and hearth smelting c-apacity being deiicient in relation to shaft raw material preparation capacity, the means above described for controlling ratio of these capacities also provides means for successful use of oxygen. With use of oxygen enriched air more heat will be generated per pound of air than with natural air but not any more per pound of carbon burned, consequently, the coke combustion rate and stock travel through the furnace will both be increased. Using Table No. 2B operation as an exampleif 9.92% more hearth heat were generated with use of oxygen-enriched air, the coke combustion rate and stock travel rate also would increase 9.92%, but, since, there is an indicated increase of 15% in preparation capacity, means are provided for faster material preparation without disturbance to thermal equilibrium between shaft work and bosh and hearth work.

For the great majority of furnace operations using soft earthy ores, relation between preparation and smelting capacities must to some degree approximate the thermal conditions within the furnace indicated in dot-dash lines in Fig. 3, and with such conditions shaft preparation capacity is deficient in relation to bosh and hearth smelting capacity. Consequently, when attempt is made to use oxygen enriched air, without my improved method, the increased volume of heat produced per pound of air cannot be utilized because there is already an excess of hearth heat availablewitness the relatively low blast temperature used in most soft ore furnace operations. In short, means must be provided to consume additional hearth heat before it can be utilized.

Where chemical composition of raw materials available will not permit burdening the furnace to naturally produce the ideal thermal conditions indicated in Table No. 1 and Fig. 2, they may be approximated by combining burdening (example Tables Nos. 2A and 2B) with use of oxygen-enriched air.

The 100% dolomite slag of Fig. 4 indicates a natural unsafe margin between bosh temperature and initial fusion and tapping slag temperature requirements. In other words, it shows insufficient temperature head for continuous dependable furnace operation. Additional heat generated at the tuyeres from enrichment of the natural `air with oxygen benefits furnace operation by increasing the concentration of heat and temperature head in the bosh without injurious eifect to other phases of furnace operation. The additional heat requirement is regulated by the percent of oxygen enrichment of the natural air. With such means for providing adequ-ate protection of the temperature head relationship, additional ore burden may be used thus giving increased iron production with decreased fuel rate.

Referring to the table hereinabove giving the melting points of minerals commonly found in blast furnace slags, it will be noted that calcium bisilicate (one part calcium oxide and two parts silica) has a much lower melting point than calcium orthosilicate (two parts calcium oxide and one part silica). Therefore, an increase in the silica charged with the burden, where calcite stone is being used, will tend to cause the formation of more calcium bisilicate and less calcium orthosilicate, thus producing a lower melting point slag as I have taught hereinabove.

The benets to be derived from the operating practice methods presented hereinabove are rst, the control of the relation between raw material preparation and smelting capacities of the furnace by controlling the location 10 of the initial fusion plane; second, the reduction ofr carbon solution loss by reducing the depth or extent of high temperature zone in the shaft where solution loss reaction occurs; third, the removal of restriction to the use of high blast temperature which permits the recovery of.

a greater percentage of latent heat (CO) normally lost to the furnace with top gas; fourth, the provision of means for utilizing low alumina ores Without the loss to thermal` economy; and fth, the provision of means for the successful use of oxygen enriched air in iron blast furnace operation.

My invention therefore teaches the creation of a greater need for heat in the bosh and hearth of an iron producing blast furnace through (1), control of slag mineral composition; or (2) selection of raw materials having natural chemical compositions which permits a greater consumption of heat in the bosh and hearth operation than occurs in the shaft preparation operation, thus providing means for utilizing oxygen enrichment of the natural air blast and consequent advantage of fast stock travel to furnace productive capacity.

What l claim is:

1. The method of operating a blast furnace producing metallic iron chieily from iron ores, comprising continuously burdening the furnace with iron bearing materials, fluxing materials and carbonaceous solid materials with regard to the chemical composition of said materials so that the temperature head in the bosh and hearth of the furnace in relation to initial slag formation temperature and tapping slag temperature is insutiicient for continuous furnace operation using natural air blast, and then increasing heat generation with oxygen enriched air without increasing the volume of the blast entering the tuyeres.

2. The method of operating a blast furnace producing metallic iron `chiefly from iron ores, comprising continu ously burdening the furnace with iron bearing materials, lluxing materials and carbonaceous solid materials with regard to the chemical composition of said materials so that slag formation in stages of its composition prior to the tapping stage will be effected at temperatures approximating the tapping stage temperature, whereby the smelting capacity of the bosh and hearth is decient in relation to the preparation capacity of the shaft, and then adding not over 5% excess oxygen by weight to the blast whereby to increase the heat generation from carbon combustion without substantially increasing the volume of gases passing through the shaft above the bosh.

3. The method of operating a blast furnace producing iron chiefly from iron ores, comprising continuously burdening said furnace with iron bearing materials, fluxing materials and carbonaceous solid materials with regard to the chemical composition of said materials so that the slag mineral constituents provide, approximately at the lower bosh section of said furnace, a slag having a formation point temperature insuicient to assimilate the coke ash and still be free-running at tapping, and enriching the blast with oxygen, whereby increased heat volume generated at the tuyere zone due to oxygen enrichment of the blast will be substantially consumed in reactions involving slag assimilation of fuel ash released with fuel carbon combustion at the tuyeres, thus maintaining slag at the free running temperature necessary for its tapping from the furnace while at the same time avoiding excessive concentration of heat in other sections of the smelting division of work.

4. The method of operating a blast furnace producing iron chiefly from iron ores, comprising continuously burdening said furnace with iron bearing materials, fluxing materials and carbonaceous solid materials with regard to the chemical composition of said materials so that the slag mineral constituents provide, approximately at the lower bosh section of said furnace a slag having a formation point temperature insufficient to assimilate the coke ash and still be free-running at tapping, increasing the total burden on said furnace to the point where without other changes the furnace would cool in the hearth, and enriching said blast with oxygen without substantially increasing the volume of the blast to increase heat generation from carbon combustion at the tuyeres so that the furnace does not cool in the hearth and equitable division of heat volume can be maintained between the preparation for smelting division of work and the smelting division of work without disturbance of equilibrium in the mechanical activity of burden materials and gas flow through the furnace..

5. The method of operating a blast furnace producing iron chiefly from iron ores, comprising continuously burdening said furnace with iron bearing materials, uxing materials and carbonaceous solid materials with regard to the chemical composition of said materials so that the slag mineral constituents provide, approximately at the lower bosh section of said furnace, a slag having a formation point temperature not substantially more than 10 percent above slag tapping temperature, increasing the total burden on said furnace to the point where without other changes the furnace could not be operated, and enriching said blast with oxygen so that the furnace can be operated.

6. The method of operating a blast furnace producing iron chiefly from iron ores, comprising continuously burdening said furnace with iron bearing materials, carbonaceous solids and iluxing materials, wherein the burden contains magnesium oxide and calcium oxide in a ratio to each other and to the silica and alumina in the burden materials causing formation in the basicity stage slag of a greater percentage of monticellite and a lesser percentage of calcium orthosilicate, thereby lowering the natural temperature required for free-running slag in the basicity stage, and then enriching the air blast with oxygen.

7. The method of operating a blast furnace producing iron chiefly from iron ores, comprising continuously burdening said furnace with iron bearing materials, carbonaceous solids and uxing materials, wherein the burden contains magnesium oxide and calcium oxide in a ratio to each other and to the silica and alumina in the burden materials causing formation in the basicity stage slag of a greater percentage of monticellite and a lesser percentage of magnesium orthosilicate, thereby lowering the natural temperature required for free-running slag in the kil basicity stage, and then enriching the air blast with oxygen 8. The method of operating a blast furnace producing iron chiefly from iron ores, comprising continuously burdening said furnace with iron bearing materials, carbonaceous solids and uxing materials, wherein the burden contains magnesium oxide and calcium oxide in a ratio to each other and to the silica and alumina in the burden materials causing formation in the basicity stage slag of a greater percentage of calcium bisilicate and a lesser percentage of calcium orthosilicatc, thereby lowering the natural temperature required for free-running slag in the basicity stage, and then enriching the air blast with oxygen.

9. The method of operating a blast furnace producing iron chiey from iron ores comprising continuously burdcning the furnace with a mixture of iron bearing materials, carbonaceous solid materials and fluxing materials containing slag-forming constituents in ratio to each other which combine to form slag of mineral composition having a formation temperature at its highest basicity stage not substantially more than ten percent above the tapping slag temperature, and adding up to five percent above the natural oxygen content of the air blast, the carbonaceous solid materials and air blast plus oxygen being insuiiicient to superheat said slag to a temperature which would adversely affect the volume and velocity of gases leaving the smelting zone, whereby to provide smelting temperature in the lower part of the furnace while holding the temperature and hence the volume and velocity of the gases in the furnace shaft to as low a value as possible consistent with regular furnace operation.

References Cited in the file of this patent UNITED STATES PATENTS 1,640,485 Davis Aug. 30, 1927 2,051,383 Lennings Aug. 18, 1936 2,104,564 Lennings et al. Jan. 4, 1938 2,184,318 Ruzicka Dec. 26, 1939 FORElGN PATENTS 7,556 Australia May 30, 1927 460,138 Great Britain Jan. 18, 1937 OTHER REFERENCES Metallurgia, July 1944, pages 155 to 157 inc. 

8. THE METHOD OF OPERATING A BLAST FURNACE PRODUCING IRON CHIEFLY FROM IRON ORES, COMPRISING CONTINUOUSLY BURDENING SAID FURNACE WITH IRON BEARING MATERIALS, CARBONACEOUS SOLIDS AND FLUXING MATERIALS, WHEREIN THE BURDEN CONTAINS MAGNESIUM OXIDE AND CALCIUM OXIDE IN A RATIO TO EACH OTHER AND TO THE SILICA AND ALUMINA IN THE BURDEN MATERIALS CAUSING FORMATION IN THE BASICITY STAGE SLAG OF A GREATER PERCENTAGE OF CALCIUM BISILICATE AND A LESSER PERCENTAGE OF CALCIUM ORTHOSILICATE, THEREBY LOWERING THE NATURAL TEMPERATURE REQUIRED FOR FREE-RUNNING SLAG IN THE BASICITY STAGE, AND THEN ENRICHING THE AIR BLAST WITH OXYGEN. 